The present exemplary embodiment relates to printing systems. It finds particular application in conjunction with adjusting print uniformity by adjusting current to individual light emitting diodes (LEDs) in an LED printbar of a xerographic device and will be described with the particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
Light emitting diode (LED) full width array imagers are commonly used as an exposure mechanism in a printer. Typically, such imagers include an arrangement of a large number of closely spaced LEDs in a linear array. By providing relative motion between the LED printbar and a photoreceptor, and by selectively energizing the LEDs at the proper times, a desired latent electrostatic image is produced on the recording member. The production of a desired latent image is usually performed by having each LED expose a corresponding pixel on the recording member in accordance with the image-defining video data information applied to the printbar through the driver circuitry. Driver circuits of the individual LEDs are selectively energized to turn the LEDs ON/OFF at fixed intervals to form a line exposure pattern on the surface of the photoreceptor. A complete image is formed by successive line exposures.
Due to manufacturing variations, aging and environmental conditions, the LED imagers output a nonuniform exposure profile. Such variations in light output expose the photoreceptor differently which results in undesirable streaks in the image.
One technique to correct the LED output uniformity is to monitor the response of individual LEDs on the print. The output of each individual LED is printed as a test pattern of single pixel wide lines parallel to the process direction. Single pixel lines are written with only one LED, so that the optical density profile of that line is related to the corresponding LED's exposure profile. The optical density profile can be measured by scanning the test print. The lines are well separated so that changes in the reflectance profile of one line do not influence a measurement of the reflectance profile of another line. A metric derived from the optical density profile of each line, such as the line width, is correlated to that LED's exposure. The current supplied to each LED in the LED print bar is adjusted until the width of each line is within a predetermined threshold or target width.
The width of the printed line depends on the total intensity of the LED. However, the width of the printed line also depends on the spatial profile of the spot emitted from the LED and focused on the photoreceptor. Beams of the same total intensity may give different line widths because one beam has a narrow profile and the other beam has a wide profile. Therefore, adjusting the LED currents to give uniform line widths may result in different integrated intensities.
In a typical printer, the beams from two neighboring LEDs overlap. The exposure profile is a sum of the individual exposure profiles. However, because of nonlinearities in the xerographic process, the profile of the developed toner on the paper for the double pixel wide line is not necessarily equal to the sum of the two developed toner profiles for the single pixel wide lines. In other words, different pairs of LEDs that individually produce lines of equal widths can produce double pixel wide lines of different widths.
Typically, a halftone strip is composed of a screen of dots. Most of the dots are written by two or more adjacent LEDs. For the same reason different sets of two pixel wide lines can be different in width, halftone dots printed with different sets of LEDs do not necessarily have to be equal in size, even though the individual lines that they print all have the same width.
Another approach to achieve cross process uniformity is to monitor the uniformity of one or more halftone strips and adjust the LED currents to increase the uniformity of the halftone strips. In this approach, however, it is problematic to determine which LED must be adjusted to force a change in density of a narrow streak. Specifically, if a particular halftone dot is too light, and that halftone dot is written by two or more LEDs, it is not possible without additional information to determine which LED must be adjusted to increase the density of that halftone dot in a way that does not degrade the appearance of other halftone screen or line art images.
There is a need for methods and apparatuses which overcome the aforementioned problems and others.